1. Field of the Invention
The present invention relates to arrangements in which a layer of material is formed on a surface, so that the material is introduced into holes or trenches in the surface. It is particularly, but not exclusively, concerned with arrangements in which that surface is a surface of a semiconductor wafer (or substrate used for integrated circuits).
2. Summary of the Prior Art
There are a number of situations during the formation of a semiconductor device in a semiconductor wafer where it is necessary to deposit a layer onto the wafer. One such situation arises when conductive or semiconductive tracks are to be formed over the wafer, so that those tracks may make contact with active regions of the device or circuit. Normally, such tracks must then extend through an insulating layer on the surface of the wafer so as to make contact with active regions below that insulating layer, or with further conductive tracks below that insulating layer (when the holes are usually called "vias"). Where the track extends through a hole in this way, it is important that the amount of material e.g. metal filling that hole is sufficient to ensure good electrical contact.
Another situation is when an electrically insulating layer is to be formed over the wafer, in order to isolate active regions and/or conductive tracks from each other, or to form a protective covering known as a passivation layer. Such a layer is often required to cover conductive tracks or other structures on the wafer, and these structures may be close to each other so that the gaps between them form narrow trenches. It is important that the insulating material covers all the surface with sufficient thickness to provide good electrical insulation, and that the top surface of the insulating layer shall be sufficiently smooth for the next stage of wafer processing.
The normal way of forming layers on the surface of a semiconductor wafer, is by the use of a deposition technique, such as sputtering for conductive layers, or chemical vapour deposition for insulating layers. In such a technique, the surface on which the layer to be formed is bombarded with particles of the material to be deposited until a layer of a suitable thickness has been achieved.
Where that surface is the surface of a layer with a hole or trench therein extending to the surface of the wafer, the particles of the material are deposited on the sides and base of the hole or trench, but it has been found that there is a tendency for the particles to be deposited primarily at the mouth of the hole or trench, so that the width of the mouth is reduced as the deposition continues. The effect of this is that the interior of the hole or trench may suffer from shadowing, and a suitably thick layer of the material may not be deposited inside the hole before deposition at the mouth of the hole or trench effectively closes the hole or trench and prevents further deposition therein, or before the required thickness has already been deposited elsewhere on the surface. This problem becomes increasingly significant as the width of the structure decreases, and developments in semiconductor technology have resulted in moves towards smaller and narrower structures.
An alternative method of producing a suitable conductive layer is firstly to fill the hole with one metal, and then form the metal layer over the insulation and the filled hole. Thus, the hole may be filled with tungsten using a technique such as chemical vapour deposition, and when a more common metal, such as aluminium or aluminium alloy may be deposited over the surface by the sputtering technique discussed above. However, the gaseous sources for the materials used to fill the holes by chemical vapour deposition are expensive, and a two-stage process involving different materials is necessary, increasing the cost of the whole device.
Holes can be filled by sputtering at high temperature (&gt;500.degree. C.) and/or using bias sputtering, but the quality of the metal is degraded, and the process is inconsistent and hard to control. Aluminium CVD is possible and does fill holes, but the process is slow, hard to control, and requires previous deposition of a suitable seed layer. Again, a two-stage process involving different materials is then necessary.
There are alternative methods of producing a suitable insulating layer. One method is to deposit part of the required thickness by chemical vapour deposition (CVD), and then to remove the parts of the layer that overhang the trench by sputter etching or reactive ion etching. This cycle may be repeated until sufficient thickness has been deposited, the etching steps being used to prevent the closing of the mouth of the trench. However the process is slow, requires several steps, and must be adjusted for different geometries.
Another method is to deposit an insulating material that can be reflowed by melting, such as silicon oxide doped with boron or phosphorous. The material may be deposited by CVD, and then heated until it flows into the trench. However the temperature required for reflow of such material is greater than 800.degree. C., which will cause melting of any aluminium tracks present, and can cause undesirable diffusion in active regions of devices in the water.
A third method is to apply a liquid solution onto the surface of the wafer, where such liquid when subsequently heated forms a solid insulating layer, such as that known as "spin-on-glass". The material flows into the trenches when first applied. However, the material tends to retain some moisture after the heating process, and this moisture can cause device unreliability due to corrosion. It may require a capping layer to seal against moisture, which increases the number of process steps and hence the device costs.